It is standard to rate the static load that can be carried by a steel beam at ambient temperature, and to fireproof it in the field by spraying or otherwise cladding the installed steel with concrete. Such covering with concrete before installation is ruled out since it is essential to be able to bolt together faces of the steel of the beam for dimensional as well as structural accuracy. Precoating with concrete would make the structural elements impossible to dimension accurately, since the sprayed coating cannot be made as accurately as the steel beam itself unless done in a mold.
In recent times it has been suggested to make a fireproof structural element by filling a longitudinal channel of the beam in question with concrete and even stabilizing this concrete with reinforcing bars. Thus, as described in German patent document No. 2,829,864, the channels of an I- or H-beam are completely filled with concrete, flush with the edges of the flanges, and while leaving the outer faces of these flanges fully exposed. In order to prevent differential thermal expansion from separating the concrete from the beam in a fire, it is standard to provide connectors welded to the beam web so that the concrete and beam are solidly locked together. This concrete, in which steel reinforcing bars are imbedded, does not project beyond the planes defined by the outer edges of the flanges, so the outline, that is the outer dimensions of the thus fireproofed beam, remains that of the basic I- or H-beam, greatly easing subsequent installation.
In a fire the exposed beam flanges are heated first, so that, although under normal circumstances they bear most of the load, they weaken and the load is transferred to the reinforced-concrete portion of the composite element. In addition in a fire the steel reinforcement of the concrete is normally positioned so that it also is heated and softens rather rapidly. Thus it is necessary to make the composite beam relatively massive and correspondingly expensive to obtain the desried fire rating.
Another disadvantage of the known such composite beam is that its fabrication is fairly complex, requires excessive effort in the field. Thus the heavy beams must be transported to the job from a remote shop.
Accordingly in commonly owned U.S. Pat. No. 4,571,913 a composite structural element is described having a main steel beam having a web and at least two flanges extending therefrom, having oppositely directed outer faces, having outer edges generally defining a plane and defining with the web a recess open away from the web between the outer edges. A mass of concrete fills the recess substantially to the plane, the outer flange faces being exposed and substantially free of concrete. Another profiled steel beam is fixed to the web of the main beam and is wholly imbedded in and covered by the concrete mass. Typically according to this earlier invention the main beam is an H- or I-beam and has two such channels provided with such other beams and filled with respective such masses.
Such an arrangement takes longer to weaken in a fire, but is still susceptible of improvement. This is particularly a problem with columns of regular polygonal section which are most attractive and convenient to use when all of their critical outer surfaces are formed of the flat flanges of the steel profile elements imbedded in the concrete.